chore: add async_rest extra for async rest dependencies

[parthea]

This PR adds an extra for async_rest which captures the minimum version of google-auth which is required to use the async_rest feature.

[vchudnov-g]

One non-trivial comment about using the extras in setup.py

[ohmayr]

One non-trivial comment about using the extras in setup.py

What's the comment? I see that we're setting extras in setup.py here

[vchudnov-g]

I suggest restructuring just slightly for brevity (I marked with ## the sections I changed). This depends on pip install foo[] with an empty extras list working, which I think it does. If not, one more line will be needed.

    install_extras = []    ##
    if install_grpc:       ##
        install_extras.append("grpc")  

    constraints_dir = str(CURRENT_DIRECTORY / "testing")
    if install_async_rest:   ##
        install_extras.append("async_rest")
        constraints_type = "async-rest-"
    else:
        constraints_type = ""

    if prerelease:
        install_prerelease_dependencies(
            session,
            f"{constraints_dir}/constraints-{constraints_type}{PYTHON_VERSIONS[0]}.txt",
        )
        # This *must* be the last install command to get the package from source.
        session.install(
            "-e", f".[{','.join(install_extras)}]", "--no-deps" ## see my other comment about installing grpc
        )
    else:
        constraints_file = (
            f"{constraints_dir}/constraints-{constraints_type}-{session.python}.txt"
        )
        # fall back to standard constraints file
        if not pathlib.Path(constraints_file).exists():
            constraints_file = f"{constraints_dir}/constraints-{session.python}.txt"

        session.install(
            "-e",
            f".[{','.join(install_extras)}]",  ##
            "-c",
            constraints_file,
        )

[ohmayr]

addressed.

[vchudnov-g]

This was a pre-existing issue: at the top of the function we require that install_grpc be set if prerelease is set. But here we don't install grpc. Is that an oversight? It seems to me we probably should install it here.

[ohmayr]

addressed.

[vchudnov-g]

Add a reminder at the top of the else that the constraints file does not determine whether the listed imports are imported.

[ohmayr]

addressed.